The present invention relates to a control method for an electromagnetic actuator for the control of a valve of an engine.
As is known, internal combustion engines of the type disclosed in Italian Patent Application B099A000443 filed on Aug. 4, 1999, are currently being tested, in which the intake and exhaust valves are moved by electromagnetic actuators. These electromagnetic actuators have undoubted advantages, as they make it possible to control each valve according to a law optimised for any operating condition of the engine, while conventional mechanical actuators (typically camshafts) make it necessary to define a lift profile for the valves which represents an acceptable compromise for all the possible operating conditions of the engine.
An electromagnetic actuator for a valve of an internal combustion engine of the type described above normally comprises an actuator body, which is connected to the stem of the valve and, in rest conditions, is held by at least one spring in an intermediate position between two de-excited electromagnets; in operation, the electromagnets are controlled so as alternately to exert a force of attraction of magnetic origin on the actuator body in order to displace this actuator body between the two limit abutment positions, which correspond to a position of maximum opening and a position of closure of the respective valve.
When the engine is off, the electromagnets are de-excited, and the actuator body is in the above-mentioned intermediate position under the action of the elastic force exerted by the spring; when the ignition of the engine is requested, the actuator body must initially be brought into a limit abutment position against an electromagnet corresponding to the closed position of the respective valve. However, neither of the two electromagnets is able to exert a force sufficient to displace the stationary actuator body, i.e. lacking kinetic energy, from the intermediate position to the abutment position; for this reason, the electromagnets are actuated alternately in order to generate an oscillating movement of the actuator body about the intermediate rest position, which oscillating movement is progressively amplified in order to cause the actuator body to come into abutment against the desired electromagnet.
In known electromagnetic actuators, the control of the electromagnets in order to bring the actuator body from the intermediate rest position to the desired abutment position takes place as an open loop, by supplying the electromagnets with respective current waves whose duration and intensity are predetermined during the actuator design stage. It has been observed, however, that the open loop control during the above-mentioned stage of actuation of the electromagnetic actuator has various drawbacks, due chiefly to the dispersion and the drift over time of the characteristics of the actuator, and the variation of the characteristics of the actuator with temperature variations. It has in particular been observed that the open loop control during the stage of actuation of the electromagnetic actuator leads in some conditions to a failure to achieve the desired condition of abutment (or to the achievement of this condition of abutment in very long periods of time) and leads, in other conditions, to the achievement of the desired abutment condition with a speed of impact of the actuator body against the electromagnet which is relatively very high, with a resultant increase both in the mechanical stresses on the electromagnetic actuator and in the noise generated by this electromagnetic actuator.
In order to attempt to remedy the above-described drawbacks, it has been proposed to use an external position sensor, which provides, instant by instant, the exact position of the actuator body and makes it possible precisely to control the actual position of the actuator body; position sensors able to provide the precision and service life needed for profitable use for this purpose are not, however, commercially available.
The object of the present invention is to provide a control method for an electromagnetic actuator for the control of a valve of an engine, which is free from the above-mentioned drawbacks and, in particular, is easy and economic to embody.
The present invention therefore relates to a control method for an electromagnetic actuator for the control of a valve of an engine as claimed in claim 1.